Pyrotechnic device for an electronic circuit

ABSTRACT

A pyrotechnic device including at least one layer of a pyrotechnic material deposited on all or part of an electronic circuit, the pyrotechnic material being capable of being triggered by a heating of the circuit due to a failure thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patent application number 15/55091, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

BACKGROUND

The present disclosure generally relates to electronic circuit and, more particularly, to an electronic power component or circuit equipped with a pyrotechnic security.

DISCUSSION OF THE RELATED ART

In power electronic circuits, power components (power transistors, diodes, thyristors, triacs, etc.) having automated or controlled switching functions are used. Such components can for example be found in motor control inverters (typically, in an electric vehicle), power converters, as well as in power cutout switches such as circuit breakers, static contactors, etc.

In case of a failure of a power switch, it is particularly important for this failure to take the form of an opening of the circuit (interruption of the connection performed by the power component) and not of a short-circuit. Indeed, a short-circuit failure may take the form of a short-circuiting of the system power source and generate very high currents of several thousands of amperes. Such currents generate overheatings such that the system may catch fire, which is particularly dangerous.

The use of a fuse between the power source and the electronic power system sometimes appears to be insufficient.

The use of pyrotechnic elements has already been provided to cause an opening of an electric circuit and guarantee the opening in open circuit and not in short-circuit in case of a failure resulting in too high a current. Document FR-A-3005782 describes a disconnector for a DC electric power supply source, based on an opening of a contact by pyrotechnic triggering. The explosion of the pyrotechnic element is controlled by an external control element and the gases generated by the explosion spread in an expansion chamber and actuate a movable blade which breaks the electric conductor to be disconnected.

Document EP-A-1709335 describes a pyrotechnic microsystem where a pyrotechnic material is deposited on a substrate. The explosion of the pyrotechnic material generates a quantity of gas causing an actuation. The initiation of the pyrotechnic material is caused by a heating resistive track or wire where the flowing of a current is triggered by a control center.

Document EP-A-1344744 describes a pyrotechnically actuated microvalve based on the triggering of pyrotechnic material deposited on an electric resistor.

In all such systems, an external control element causes the triggering of the pyrotechnic charge.

SUMMARY

An embodiment overcomes all or part of the disadvantages of usual electronic power circuits.

An embodiment provides a solution avoiding a short-circuit failure of a power component.

An embodiment provides an improved pyrotechnic solution.

Thus, an embodiment provides a pyrotechnic device comprising at least one layer of a pyrotechnic material deposited on all or part of an electronic circuit, the pyrotechnic material being capable of being triggered by a heating of the circuit due to a failure thereof.

According to an embodiment, the breaking of one or a plurality of conductors is caused by the pyrotechnic effect.

According to an embodiment, the explosion of the pyrotechnic material causes the breaking of all the conductors electrically in parallel.

According to an embodiment, the device further comprises at least one rigid element capable of being propelled towards the conductor(s) by the pyrotechnic gases.

According to an embodiment, the thickness of the pyrotechnic material is in the range from 10 to 100 micrometers.

According to an embodiment, the pyrotechnic material is electrically insulating.

An embodiment also provides an electronic device comprising:

an electronic circuit; and

a pyrotechnic device.

According to an embodiment, the device further comprises a package at least partially covering the electronic circuit and the pyrotechnic device.

According to an embodiment, the package is formed of an encapsulation material covering the circuit and the pyrotechnic device and at least partially embedding the conductor(s).

According to an embodiment, the package is hollow.

According to an embodiment, the electronic circuit integrates a power switch.

An embodiment provides a method of forming a pyrotechnic device, wherein a pyrotechnic material is deposited in the liquid state on the electronic circuit.

The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show, respectively in perspective and in cross-section view, and very schematically, an embodiment of an electronic power circuit with a pyrotechnic security device;

FIGS. 2A and 2B are partial cross-section views illustrating the triggering of the pyrotechnic security according to an embodiment of a power circuit;

FIG. 3 is a simplified top view of another embodiment of a power switch with a pyrotechnic security;

FIG. 4 is a simplified cross-section view of still another embodiment of a power circuit with a pyrotechnic security, illustrating different variations; and

FIG. 5 shows, in a simplified cross-section view, still another embodiment of a power switch with a pyrotechnic security.

DETAILED DESCRIPTION

For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and will be detailed. In particular, the forming of the actual power component (transistor, thyristor, triac, diode, etc.), that is, of the active portion thereof, for example, made of a semiconductor material, has not been detailed, the described embodiments being compatible with usual technologies for manufacturing power components (semiconductor or not). Further, the applications of the electronic circuit integrating the power component have not been detailed, the described embodiments being here again compatible with usual applications of power components.

It should be noted that, in the drawings, the structural and/or functional elements common to the different embodiments may be designated with the same reference numerals and may have identical structural, dimensional, and material properties.

Unless otherwise specified, expressions “approximately”, “substantially”, and “in the order of” mean to within 10%, preferably to within 5%.

The present disclosure will refer to an example of application to an electronic circuit formed of a semiconductor power component. It should however be noted that the embodiments which will be described more generally apply to any electronic circuit integrating one or a plurality of power components.

FIG. 1A and 1B show, respectively in perspective and in cross-section view, and very schematically, an embodiment of a power circuit with a pyrotechnic security device.

A power component 2 is for example supported by an electronic board 1 (for example, a printed circuit board PCB). Power component 2, for example, a diode, a transistor, a thyristor, a triac, etc. is formed in a semiconductor substrate (for example, made of silicon) and comprises electric contacts 22 intended to receive conductors, for example, wires 32, of electric connection to other elements (not shown) of the circuit or of the electronic system.

It is provided to add to the power component a pyrotechnic security capable of breaking one or a plurality of electric connection conductors in case of a failure of the component.

A specificity of such a pyrotechnic security is that the explosion is initiated by the actual failure without requiring an external control element.

According to the embodiment shown in FIGS. 1A and 1B, the pyrotechnic security device comprises a layer of a pyrotechnic material 42 deposited on component 2. The function of the pyrotechnic material is to cause a breakage of at least one conductor 32, under the effect of the gases generated on explosion thereof. If a plurality of conductors are electrically connected in parallel, the pyrotechnic material causes the breaking of all these conductors to guarantee a breaking of the electric connection.

FIGS. 2A and 2B are partial cross-section views illustrating the triggering of the pyrotechnic security according to an embodiment of a power component. As an example, FIGS. 2A and 2B are considered as showing an enlargement of FIG. 1B at the level of the active (semiconductor) area of power component 2 formed in the semiconductor substrate. FIG. 2A illustrates the triggering of the pyrotechnic security and FIG. 2B illustrates the action thereof on one of conductive wires 32.

In normal operation, the current generally in the range from some ten amperes to a few hundreds of amperes according to circuits, which flows through active area 24 of the component generates no temperature rise capable of making the pyrotechnic material explode.

On occurrence of a failure in the component (for example, breakdown of a dielectric, crack in the component, etc.), the current conducted by conductor(s) 32 and flowing through the component tends to concentrate in the area of the defect since the resistance thereof is lower than in the rest of the component. This locally generates a current increase which causes the melting of the materials (metal, silicon, etc.) of this area, thus decreasing the local resistance and causing a runaway effect. The hot spot which creates and the local melting of the materials cause a triggering of the security by the explosion of the pyrotechnic material. The gases caused by the explosion then break conductive wire 32 (FIG. 2B) or the conductive wires in parallel.

Thus, the security acts on (and destroys) its own triggering element (conductive wire(s) 32). Accordingly, this guarantees an opening of the electric circuit even if, at the level of component 2, the failure has taken the form of a short-circuit. The mechanical breaking of conductor 32 or of the conductors electrically in parallel guarantees that the failure results in an open circuit. In the case of series-connected conductors, it is sufficient to break the series connection.

It should be noted that the components and the electronic circuit are then destroyed (the security is not resettable). This is however the desired effect in the targeted applications.

Pyrotechnic material 42 is preferably present at least at the level of conductors 32 to be broken by the pyrotechnic action. This enables to improve the effect thereof.

Further, to optimize the action of the gases, the assembly is preferably placed in a closed enclosure, as will be seen hereafter in relation with FIG. 4.

The pyrotechnic material is preferably selected to have all or part of the following properties:

To be an electric insulator, since it covers or joins different conductive areas. More specifically, the leakage current generated by the pyrotechnic material should not exceed that of the component itself.

Not to trigger at homogeneous temperatures corresponding to the component operating temperatures. Preferably, a pyrotechnic material triggering at temperatures greater than approximately 150° C. and preferably greater than approximately 300° C. will be selected. The inventor has indeed observed that a failure generally results in a hot spot at more than approximately 500° C. The fact for the pyrotechnic material not to trigger below 500 or 600° C. has the advantage that it enables to deposit it on the component before certain component soldering steps which generally generate temperatures in the range from 300 to 400° C., which would otherwise risk untimely triggering the security.

For the combustion residue of the pyrotechnic layer to be sufficiently insulating to avoid generating a significant leakage current.

To be triggerable by a hot spot having a surface area of a few square micrometers within less than from a few hundred milliseconds to a few hundreds of microseconds.

Preferably, the integration of the pyrotechnic material is performed at the end of the manufacturing of the component or of the circuit, after the welding of conductors 32 on contacts 22.

The material is preferably deposited in liquid or paste form to spread on the component and insert above conductor(s) 32. The pyrotechnic material is for example deposited with a thickness of a few micrometers.

As a specific embodiment, a nitrocellulose varnish which ignites at a temperature in the range from 160 to 170° C. may be used, provided for this to be compatible with the subsequent manufacturing steps (for example, case of a cold solder for subsequent steps) and particularly with the encapsulation of the component.

According to another example, the material is a polymer from the family of glycidyl azide polymers (GAPs) which have an ignition temperature of approximately 573° C.

According to the described embodiments, it is provided to cover the entire component or only certain areas thereof.

FIG. 3 is a simplified top view of another embodiment of a power component with a pyrotechnic security.

According to this embodiment, pyrotechnic security device 4 comprises, in addition to pyrotechnic material layer 42, a rigid intercalary element 44 between pyrotechnic material 42 and conductor(s) 32. Element 44 is used as a projectile when pyrotechnic material 42 explodes and eases the breaking of the conductors. Preferably, element 44 is arranged on the pyrotechnic material, for example, glued (which is easy if the pyrotechnic material is deposited in liquid varnish form) and is located on the conductor(s) to be disconnected. Element 44 may be one piece, as shown in FIG. 3, or may be formed of a plurality of projectiles. Preferably, element 44 is insulating to avoid any risk of creating a short-circuit between conductors 32 in normal operation.

FIG. 4 is a simplified cross-section view of still another embodiment of a power component with a pyrotechnic security, illustrating different variations.

This embodiment features a package 6 covering the electronic circuit and particularly power component 2.

In the example of FIG. 4, component 2 is assumed to be connected, by one or a plurality of conductive tracks 34, for example, made of copper, to a connection tab 35 emerging outside of package 6. For example, track 34 forms a ground plane of the power component or a rear surface electrode. Further, inside of the package, an upper contact of component 2 is connected by a conductive wire 32 to a copper track 36 of circuit 1 connected to another connection tab 37.

In the example of FIG. 4, pyrotechnic material 42 is only deposited on the component and on copper track 34 to surround tab 35 (and pass under tab 35 when the latter comprises horizontal portions internal to package 6). Thus, on activation of pyrotechnic material 42, tab 35 and wire 32 are broken.

In the example of FIG. 4, a rigid intercalary element 44 is also provided between layer 42 and wire(s) 32.

It should be noted that, in embodiments providing a rigid element used as a projectile, package 6 is empty, at least in the vicinity of this element to enable the projection thereof.

According to a variation, not shown, the package is full. For example, the assembly of component 2, material 42 and wire 32 is embedded in encapsulation resin (for example, epoxy resin). In this case, an intermediate element 44 has no effect. However, the fact for wire 32 to be embedded in resin results in that, when a pyrotechnic triggering destroying the package occurs, wire 32 embedded in resin is effectively torn off.

According to a variation, illustrated in FIG. 4, to ease the breaking of tab 35, the latter comprises a weakened area 352, for example, a notch. The notch should however be such that tab 35 keeps a section capable of conducting the normal operation current. The crossing of package 6 by tab 35 may possibly be a rigid connection. This contributes to easing the breaking when the pyrotechnic effect takes place.

According to another variation, not shown, pyrotechnic material 42 extends all over circuit 1 inside of package 6.

FIG. 5 shows, in a simplified cross-section view, still another embodiment of a power component with a pyrotechnic security. According to this embodiment, two circuits 1 and 1′ are connected by a ball grid array (BGA). The interval between balls and, preferably, between circuits 1 and 1′, is filled with pyrotechnic material. Thus, this material is used as an encapsulation for the component while ensuring the protection thereof.

An advantage of the embodiments which have been described is that the security is particularly fast to trigger.

Another advantage is that the absence of a control circuit guarantees a reliable actuation. Indeed, since the triggering is exclusively thermal, no electronic failure risks preventing this triggering.

Another advantage is that the security device is particularly easy to form.

As a specific embodiment, the thickness of the pyrotechnic material is in the range from 10 to 100 micrometers.

Various embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. In particular, the practical implementation of the described embodiments is within the abilities of those skilled in the art based on the functional indications given hereabove and by using existing pyrotechnic materials.

Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto. 

What is claimed is:
 1. A pyrotechnic device comprising: one or a plurality of conductors; and at least one layer of a pyrotechnic material deposited on all or part of an electronic circuit, the pyrotechnic material being capable of being triggered by a heating of the circuit due to a failure thereof, the pyrotechnic effect causing the breaking of said conductor(s).
 2. The device of claim 1, wherein the explosion of the pyrotechnic material is triggered by a hot spot at the level of the conductor(s).
 3. The device of claim 1, having a plurality of said conductors electrically in parallel, the explosion of the pyrotechnic material causing the breaking of all the conductors electrically in parallel.
 4. The device of claim 1, further comprising at least one rigid element capable of being propelled towards the conductor(s) by the pyrotechnic gases.
 5. The device of claim 1, wherein the thickness of the pyrotechnic material is in the range from 10 to 100 micrometers.
 6. The device of claim 1, wherein the pyrotechnic material is electrically insulating.
 7. An electronic device comprising: an electronic circuit; and the pyrotechnic device of claim
 1. 8. The device of claim 7, further comprising a package at least partially covering the electronic circuit and the pyrotechnic device.
 9. The device of claim 8, wherein the package is formed of an encapsulation material covering the circuit and the pyrotechnic device and at least partially embedding the conductor(s).
 10. The device of claim 8, wherein the package is hollow.
 11. The device of claim 7, wherein the electronic circuit integrates a power switch.
 12. A method of forming the pyrotechnic device of claim 1, wherein a pyrotechnic material is deposited in the liquid state on the electronic circuit. 